1. Field of the Invention
The invention relates to a method for optically detecting deviations of the image plane of an optical imaging system from the upper surface of a substrate to be arranged in the vicinity of that image plane and an imaging device, in particular, a projection exposure system for use in microlithography, that has an optical imaging system and a focus-detection system for carrying out the method.
2. Description of the Related Art
Projection exposure systems for use in microlithography have been employed for fabricating semiconductor devices and other types of microdevices for several decades. They serve to image patterns on masks, or reticles, onto a substrate coated with a photosensitive layer at ultrahigh resolution, on a reduced scale, using an optical imaging system, their projection lens, where the upper surface of the substrate must coincide with the image plane of the imaging system as closely as possible if imaging is to be uniformly sharp over the entire image field.
In order to create even finer features of order 100 nm and finer, projection lenses operating with ultraviolet light that have high image-side numerical apertures, NA, of, e.g., NA=0.8 or better, and correspondingly shallow depths of field are employed, which imposes particularly stringent demands on the accuracy with which the substrates to be exposed are positioned in relation to the image plane.
Alignment systems are used for positioning substrates along directions (the x-direction and y-direction) orthogonal to the optical axis. Focus-detection systems, which are also termed “focus sensors,” are used for optically detecting deviations of the substrate's upper surface from the image plane parallel to the optical axis (the z-direction) and any tilting of the substrate's upper surface. Several of those systems have an optical input-coupling system for obliquely injecting at least one measuring beam to be reflected at the substrate's upper surface into a slit-shaped intermediate expanse situated between a final optical surface of an imaging system and the substrate's upper surface and an output-coupling system for detecting the measuring beam following its reflection at the substrate's upper surface. In the case of systems of this type, the measuring beam is incident on the substrate's upper surface at an acute angle of incidence and reflected. The measuring beam, which is detected and may be utilized for determining the position and, if necessary, the orientation, of the substrate' upper surface, will be offset by an amount that will depend upon the height of the substrate's upper surface in the z-direction and any tilting of the substrate's upper surface.
U.S. Pat. No. 5,783,833 describes a focus sensor of that type that employs grazing incidence of the light employed for measurement. However, the problem that, in the case of imaging systems with high numerical apertures and correspondingly short working distances between their exit surfaces of their lenses and the substrate's upper surface, not much space remains available, so that a focusing beam cannot be directed at the substrate's upper surface at a sufficiently large angle of incidence, is pointed out. It is therefore assumed that focus-detection systems employing light incident at oblique angles of incidence may be employed on projection lenses having moderate numerical apertures and correspondingly large working distances only.
U.S. Pat. No. 5,191,200 describes other microlithographic projection lenses having focus and tilt sensors. In addition to embodiments wherein light incident at grazing angles is reflected at the substrate's upper surface only, there also are embodiments wherein a transparent reference plate whose exit surface is utilized as a reflecting surface for light employed for measurement that is obliquely incident from below is arranged beneath the final lens element of the projection lens. These focus-detection systems also require relatively large working distances between the final optical surface of the imaging system and the upper surface of the substrate to be positioned.
Furthermore, there are focus sensors that utilize the wave nature of light. Measuring beams are normally incident outside that area of the substrate to be exposed, whose location in the z-direction is determined using interference.